As is well known in the art, a television picture comprises a video frame recurring at a rate of about 30 times per second (for black-and-white images) and containing roughly 500 lines of as many picture elements (pixels) each. For reasons of personal privacy, commercial interest or national security it is often desirable to transform such a train of digitized video signals--or, for that matter, any other train of data words--into a form which does not enable ready reconstruction of the original information content by an interceptor. This technique, known as "scrambling", requires a certain transposition of the data words within the train in a manner known to an authorized receiver who uses the converse of that procedure to "unscramble" the arriving modified train so as to restore the data words to their original order of succession.
Such a transformation and retransformation requires the use of a memory capable of accommodating all the data words involved in the scramble. In video transmission, in particular, it would be convenient to carry out the transposition among a recurrent set of Z frames which may be considered a 3-dimensional structure X.multidot.Y.multidot.Z, with X denoting the number of pixels per line and Y being the number of lines per frame while Z is a temporal dimension. Since the contents of the memory cells are not being read in the same order in which they are written, the use of an unvarying transposition pattern does not allow for a reloading of a cell immediately after its contents have been read out.
A simple solution for avoiding the overlapping of stored data in such a memory is an increase in the number of its cells to accommodate at least 2Z-1 frames. Such a system, however, is rather complex and correspondingly costly.
It is also necessary to bear in mind that currently available memories have access times substantially exceeding the recurrence period of video samples or pixels whose repetition frequency is on the order of 10 MHz.